1. Field of the Invention
The present invention relates to a progressive multifocal lens used to assist an accommodation power of an eye.
2. Related Background Art
As spectacle lenses for assisting an accommodation power when an accommodation power of an eye becomes too weak to see a near point, various progressive multifocal lenses have been known. A progressive multifocal lens of this type has a "farsight portion", a "nearsight portion" and a progressive region (referred to as an "intermediate portion" hereinafter) which is located between the above two portions in which a refracting power is continuously changed. When the progressive multifocal lens is actually worn as spectacles by a user, the farsight portion becomes an upper portion while the nearsight portion becomes a lower portion. In the present invention, it is to be noted that "upper portion", "lower portion", "horizontal" and "vertical" are used to point positions of the progressive multifocal lens in the state that it is actually worn by the user. For example, the lower portion of the farsight portion is a position within the farsight portion close to the intermediate portion. Also, the difference between the nearsight refracting power and the farsight refracting power is called an additional power.
In a progressive multifocal lens of this type, in general, when wide distinct vision areas (ranges with an astigmatic difference of 0.5 diopter or less) of the farsight and nearsight portions are kept and coupled by the progressive power range, lens aberrations are concentrated on side portions of the progressive power range. As a result, blurring and distortion of an image occur in these side portions. Therefore, when a user wears spectacles formed of these multifocal lenses and shifts his eyes laterally, he perceives the distortion of an image as the fluctuation of the image and has an unpleasant feeling.
In order to prevent such a problem of visual characteristics, in known progressive power lenses, various designs and evaluations have been made from a variety of viewpoints. As for the shape of a lens surface, an intersecting line (principal meridional curve) defined by a section extending along a vertical meridian vertically passing through substantially the center of a lens surface and an object-side lens surface is used as a reference line for expressing specifications such as an additional power of a lens, and is also used as an important reference line in a lens design.
As shown in FIG. 1, a progressive multifocal lens has been proposed in which an intermediate portion P and a nearsight portion N are asymmetrically arranged in consideration of the fact that a nearsight eye point (not shown) of the nearsight portion N is slightly displaced from the center toward a nasal side when the lens is worn (hereinafter referred to as an asymmetrical progressive multifocal lens).
Also, in such an asymmetrical progressive multifocal lens, a center line MM' consisting of an intersecting line of an object-side lens surface and a section passing the center OF of a farsight portion F, a farsight eye point E and the center of the nearsight portion N is used as a reference line. These reference line will be referred to as "principal meridional curves" in the present invention.
As conventional progressive multifocal lenses, there are a type in which a lens surface is formed by microscopic spherical surfaces connected continuously along a principal meridional curve over its entire line, i.e., formed by a so-called umbilical point curve and a type in which a refracting surface is formed such that the curvature .rho.m along the principal meridional curve and the curvature .rho.s in the direction perpendicular to the principal meridional curve are substantially different from each other on a portion of the principal meridional curve.
Namely, regarding the surface shape on the principal meridional curve, conventional progressive multifocal lenses can be classified roughly into a type in which the curvature .rho.m along the principal meridional curve and the curvature .rho.s in the direction perpendicular to the principal meridional curve are approximately equivalent over the entire line of the principal meridional curve to be an umbilical point form and a type in which the curvature .rho.m along the principal meridional curve and the curvature .rho.s in the direction perpendicular to the principal meridional curve are substantially different from each other in at least a portion on the principal meridional curve. The present invention is applicable to both types of progressive multifocal lenses.
Minkwitz's law carried in Optica Acta (Volume 10, No. 3, July 1963) says that it is necessary not to make at least an intermediate portion along the principal meridional curve in the umbilical form. Namely, according to Minkwitz's law, it is said that when a lens surface is in the umbilical point form over the entire principal meridional curve, the astigmatic difference of a surface refracting power of a lens is increased at a rate of double the surface refracting power along an umbilical point, making it impossible to obtain a wide distinct vision area.
As a conventional progressive multifocal lens adopting the above-mentioned Minkwitz's law, e.g., there is a lens disclosed in Japanese Patent Application Laid-Open No. 52-110646. According to Minkwitz's law, this lens is structured such that the curvature .rho.m along the principal meridional curve and the curvature .rho.s in the direction perpendicular to the principal meridional curve are substantially different from each other in an intermediate portion on the principal meridional curve. That is, a difference is provided between two main curvatures perpendicular to each other. Thereby, a distinct vision area in the intermediate portion can be enlarged.
In a progressive multifocal lens disclosed in Japanese Patent Application Laid-Open No. 56-78817, the difference between the curvature .rho.m along the principal meridional curve and the curvature .rho.s in the direction perpendicular to the principal meridional curve is increased from a farsight porion to a nearsight portion and decreased in the nearsight portion. In the lower portion of the nearsight portion, two main curvatures become equivalent. Thereby, distortion of an image in the center of the nearsight portion can be reduced.
Also, in Japanese Patent Application Laid-Open No. 59-58415, a progressive multifocal lens wherein a portion on the principal meridional curve is not in the umbilical form is disclosed. In this lens, from a center of a farsight portion located in the lower portion of the farsight portion to a peripheral region of the lens (the upper end of the lens) and from a center of a nearsight portion located in the upper end of the nearsight portion to a peripheral region (the lower end of the lens), the curvature .rho.m along the principal meridional curve are gradually increased and the curvature .rho.s in the direction perpendicular to the principal meridional curve is kept constant. Thereby, distinct vision areas of the farsight and nearsight portions are enlarged and it is possible to make the lens thinner.
Further, in a progressive multifocal lens disclosed in Japanese Patent Application Laid-Open No. 1-221722, the value of the curvature .rho.m along the principal meridional curve and the value of the curvature .rho.s in the direction perpendicular to the principal meridional curve become maximum in approximately the upper portion of a nearsight portion and decrease toward the peripheral region of the nearsight portion along the principal meridional curve. The value of the main curvature .rho.m and the value of the main curvature .rho.s are substantially equivalent from at least the lower portion of a farsight portion to approximately the center of an intermediate portion and substantially different from each other in the nearsight portion. Therefore, a wide distinct vision area is obtained in the farsight portion and vision is improved from the intermediate portion to the nearsight portion. Further, in the side regions from the lower portion of the farsight portion to the nearsight portion where lens aberrations are liable to be concentrated, the aberration density can be decreased to reduce distortion and fluctuation of an image.
Thus, in the above-structured conventional progressive multifocal lenses, wider distinct vision areas can be obtained to a certain extent by adopting Minkwitz's low.
However, the lens in Japanese Patent Application Laid-Open No. 52-110646 is merely aiming at obtaining a wide distinct vision area of the intermediate portion, and the lens in Japanese Patent Application Laid-Open No. 56-78817 is chiefly aiming at obtaining a wide distinct vision area of the nearsight portion. Also, the lens in Japanese Patent Application Laid-Open No. 59-58415 is only aiming at obtaining wide distinct vision areas of the nearsight and farsight portions. Although the lens in Japanese Patent Application Laid-Open No. 1-221722 is aiming at obtaining wide distinct vision areas of the three portions to decrease the aberration density to reduce distortion and fluctuation of an image, it is not still sufficient to obtain a wider distinct vision area over three portions of the farsight, intermediate and nearsight portions and to improve visual characteristics of the respective portions in any of the above conventional lenses.
Especially, in conventional progressive multifocal lenses, as a visual line passing the principal meridional curve in an intermediate portion generally enters a lens surface not perpendicularly but obliquely, a surface astigmatic difference is produced if the area of the intermediate portion on the principal meridional curve has an umbilical-point-like surface.
Recently, there have been requirements for the more comfortable vision of the intermediate portion. Therefore, it is required to reduce lens aberrations in side regions of the intermediate portion to avoid distortion and fluctuation of an image in those regions. Therefore, there is a tendency to lengthen the intermediate portion (the refracting power progressively changing region) along the principal meridional curve. Also, a base curve on the entire lens surface tends to be shallow or gentle. Accordingly, the angle by the visual line and the refracting surface on the principal meridional curve in the intermediate portion, specifically the tilt angle by the visual line and the normal line of the refracting surface becomes much larger, which causes occurrence of a serious astigmatic difference.
Further, recently, in consideration of the fact that a nearsight portion comes close to a nasal side when a lens is worn, asymmetrical progressive multifocal lenses in which a nearsight portion is arranged asymmetrically have been developed. In such a lens, aberrations tend to concentrate on the nasal side as compared to a temporal side, so the user feels discomfort due to distortion and fluctuation of an image.